Light-emitting diode (LED) arrays have shown great potential as a light source in liquid-crystal display (LCD) backlighting systems. When compared to other light sources such as incandescent or fluorescent light sources, LED arrays are desirable for their low-temperature performance, ease of heat-sinking, dimming range, small size, low power consumption, relatively low cost, luminous efficacy, and directional emission.
Some LCD backlights are required to emit light of a certain chromaticity and luminance. Other backlights are required to perform in multiple viewing modes, each of the modes having different chromaticity and luminance requirements. For example, an avionics LCD display may be required to perform in a daylight viewing mode as well as in a night-time viewing mode, and the luminance and chromaticity requirements for the viewing modes are vastly different from each other. In such circumstances, it would be helpful to control the luminance and chromaticity of the backlight.
One problem with adjusting the luminance and chromaticity of a backlight is that some backlights use a plurality of light sources that emit light having different luminances and chromaticities. For example, an LED-based backlight may use different colors of LEDs that, when properly mixed, produce light having a desired chromaticity and luminance. However, once the light is properly mixed, it is difficult to reduce the luminance throughout the entire dimming range while maintaining a stable chromaticity. This makes chromaticity control difficult.
It is therefore an object of the invention to provide an LCD backlighting system that can be customized to provide light with a desired chromaticity range.
It is another object of the invention to provide an LCD backlight that provides light having good color uniformity.
It is yet another object to provide an optical feedback system that controls luminance and chromaticity of an LCD backlight having light sources with different spectral outputs.
A feature of the invention is the use of pulse-width modulation techniques to isolate and measure differently-colored light sources in a backlight.
Another feature of the invention is the use of one or more detectors that detect predetermined tri-stimulus values, which are then used to determine chromaticity and luminance of emitted light.
An advantage of the invention is that commonly-available LEDs may be used to produce an LCD backlight with a customizable chromaticity.